The present invention relates to an optical amplifier device for compensating for a predetermined gain profile that is required in an optical transmission system even if any one of a plurality of laser units that constitute a pumping light source is in trouble.
In recent years, along a rapid diffusion of the Internet and a rapid increase in the number of connections between LANs within enterprises, the transmission capacity of contents data like dynamic images has increased, not only a mere increase in the number of communication calls. Consequently, this has brought about a problem of a rapid increase in data traffics. Under this situation, a WDM (wavelength division multiplex) system has been developed remarkably and has come to be widely distributed, for preventing a reduction in the communication performance attributable to the increase in the data traffics.
The WDM system realizes a large-capacity transmission of information that is a hundred times a conventional transmission capacity, with one fiber, by allocating a plurality of optical signals to waves of mutually different wavelengths. Particularly, the existing WDM system uses an erbium-doped fiber amplifier (hereinafter to be referred to as an EDFA) to achieve a wide-band and long-distance transmission. The EDFA is an optical amplifier that utilizes the following principle. Namely, when a pumping laser of a wavelength 1480 nm or a wavelength 980 nm is used to transmit a light through a special optical fiber added with an element called erbium, a light of a wavelength 1550 nm band as a transmission signal is amplified in this special fiber.
In the mean time, the EDFA is a concentration-type optical amplifier in which portions for exciting an optical signal are concentrated. Therefore, there has been a limit to this EDFA in that there is a loss of a transmission path optical fiber leading to the accumulation of noise, and the EDFA is subjected to non-linearity that becomes the cause of signal distortion and noise. Further, the EDFA makes it possible to carry out optical amplification in a wavelength band that is determined by band gap energy of erbium, and it has been difficult to obtain a wide band for realizing further multiplexing.
As an optical amplifier device that replaces the EDFA, attention has been paid to a Raman amplifier. The Raman amplifier is a distribution-type optical amplifier that uses a normal transmission line fiber as a gain medium, without requiring a special fiber like an erbium-doped fiber that is used in the EDFA. Therefore, as compared with the WDM transmission system that is based on the conventional EDFA, the Raman amplifier can improve the transmission quality.
FIG. 5 is a block diagram showing a schematic structure of a conventional Raman amplifier. In FIG. 5, the Raman amplifier is structured by an optical multiplexer 120, optical isolators 111 to 113, and a High Power Unit (HPU) 130 that are provided on a transmission line 99.
FIG. 6 is a diagram showing a structure example of the HPU 130. In FIG. 6, the HPU 130 is composed of six laser units LD1 to LD6 having different oscillation center wavelengths, and a Mach-Zehnder-type WDM coupler 131. Each of the laser units LD1 to LD6 has two Fabry-Perot type semiconductor lasers 134 having the same oscillation center wavelength. Each laser unit stabilizes a laser output of each semiconductor laser 134 with a fiber brag grating (FBG) 133. At the same time, a polarization multiplexer (PBC) 132 multiplexes the laser outputs and produces one output. The polarization multiplexing by the PBC 132 is a measure for increasing the pumping power of each oscillation center wavelength, and for reducing the polarization dependency of Raman gain. As explained above, the HPU 130 is composed of a plurality of laser units having different oscillation center wavelengths, as it is necessary to amplify the signal light of a plurality of multiplexed wavelengths (channels)
Laser outputs obtained from the laser units LD1 to LD6 are further multiplexed by the WDM coupler 131, and a high-output multiplexed pumping light is output. The pumping light output from the HPU 130 is transmitted through an optical fiber of the transmission line 99 via the optical multiplexer 120. FIG. 5 shows an example of a backward pumping. A pumping light multiplexed by the optical multiplexer 120 is transmitted through the transmission line 99 to a direction opposite to the proceeding direction of the signal light.
When the high-output pumping light is transmitted through the transmission line 99, a Raman scattered light shifted to a long wavelength side by 110 nm from the pumping light is generated, based on material characteristics of the optical fiber of the transmission medium. Then, through an induction Raman scatter process, the energy of the pumping light is shifted to the signal light. Based on this, the signal light is amplified.
As explained above, a Raman amplifier is an amplifier capable of amplifying a signal light as it is, using an established optical fiber as an amplification medium. The Raman amplifier is different from the EDFA in the aspects of am amplification medium, a number of pumping light sources used, and pumping power. For the light source for exciting an erbium-doped fiber amplifier in the EDFA, it is also possible to use a one having a similar structure to that of the HPU 130.
It is an object of this invention to provide an optical amplifier device capable of sustaining an optical transmission without deteriorating signal transmission quality, even if any one of a plurality of laser units that constitute a pumping light source is in trouble.
In order to achieve the above object, according to one aspect of the present invention, there is provided an optical amplifier device that comprises a pumping light source for multiplexing a plurality of laser lights having different oscillation center wavelengths and outputting a pumping light obtained, and that amplifies a signal light propagated on a transmission line in a desired bandwidth and with a desired gain determined by the pumping light, wherein, when the output of at least one of the plurality of laser lights has been stopped, the optical amplifier device alters the power of at least one of laser lights other than the stopped laser light so as to secure the amplification in the desired bandwidth and with the desired gain.
Further, according to another aspect of the invention, there is provided an optical amplifier device comprising, a pumping light source that includes a plurality of laser light sources for outputting laser lights having mutually different oscillation center wavelengths, and a monitoring section for detecting output power of each laser light source, and that multiplexes laser lights output from the plurality of laser light sources, and outputs an obtained multiplexed light as a pumping light, a control unit which controls the output power of the plurality of laser light sources according to an input control signal, and a decision unit which specifies a laser light source of which output is to be stopped out of the plurality of laser light sources, based on the output power detected by the monitoring section, selecting a gain profile in the case of the specified laser light source being in a stopped status, from among a plurality of gain profiles stored in advance, and outputting output power information of each laser light source shown by the selected gain profile, as the control signal.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.